Communication system

ABSTRACT

A communications system is described in which user devices are allocated sub-carriers on which to transmit uplink data to a base station. ACK/NACK messages for the data transmitted on the uplink are then transmitted by the base station on sub-carriers that depend on the sub-carriers used to carry the uplink data. A direct mapping function is preferably used to determine the sub-carriers to be used for the ACK/NACK messages from the uplink sub-carriers. In another embodiment, the ACK/NACK messages are transmitted to the user devices on sub-carriers that are previously identified to the user devices, preferably by transmitting one or more index values to the user device in a control channel thereof.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation application of U.S. patentapplication Ser. No. 12/308,647, filed Dec. 19, 2008, which is based inon International Application No. PCT/JP2007/062370, filed Jun. 13, 2007,which claims priority from UK Patent Application No. 0705341.6, filedMar. 20, 2007, which claims priority from UK Patent Application No.0612228.7 filed Jun. 20, 2006, the entire contents of which areincorporated herein by reference.

TECHNICAL FIELD

The present invention relates to the signalling of ACK/NACK messages ina communications method and apparatus. The invention has particular,although not exclusive relevance to the signalling ACK/NACK messages inan orthogonal frequency division multiple access (OFDMA) communicationsystem.

BACKGROUND ART

OFDMA and single carrier FDMA have been selected as the downlink anduplink multiple access schemes for the E-UTRA air interface currentlybeen studied in 3GPP (which is a standard based collaboration looking atthe future evolution of third generation mobile telecommunicationsystems). Under the E-UTRA system, a base station which communicateswith a number of user devices allocates the total amount oftime/frequency resource (depending on bandwidth) among as manysimultaneous users as possible, in order to enable efficient and fastlink adaptation and to attain maximum multi-user diversity gain. Theresource allocated to each user device is based on the instantaneouschannel conditions between the user device and the base station and isinformed through a control channel monitored by the user device.

When data is transmitted from the user device to the base station, anacknowledgment (ACK) or a non-acknowledgment (NACK) is typicallysignalled back from the base station to the user device. Under thecurrent proposals for E-UTRA, these ACK/NACK messages are to be sent inthe downlink control channel for the user device. However, the inventorhas realised that this leads to a problem that the size of the controlchannel will vary depending on the situation of the user device.

DISCLOSURE OF INVENTION

According to one aspect, the present invention provides a communicationmethod, typically performed in a base station which communicates with aplurality of user devices using a plurality of sub-carriers, the methodcomprising: receiving uplink data from a user device and generating acorresponding ACK/NACK message for the received data; forming controldata defining an allocation of said sub-carriers for the user devices;transmitting said control data to the user devices; and transmittingsaid ACK/NACK message to the corresponding user devices; wherein saidcontrol data is transmitted over a control channel using a first subsetof said sub-carriers and said ACK/NACK message is transmitted on anACK/NACK channel that is separate from said control channel using asecond different subset of said sub-carriers.

Preferably the sub-carriers are grouped into a sequence of chunks orresource blocks (RBs) and the control channel allocates one or morechunks of sub-carriers to each of the plurality of user devices. In oneembodiment, an ACK/NACK message is generated for the data received oneach chunk of sub-carriers.

Preferably the sub-carriers to be used to transmit an ACK/NACK messageto a user device are determined in dependence upon the sub-carriersallocated to that user device for transmitting the uplink data that isbeing acknowledged. This avoids the need for the base station toseparately signal data to each user device identifying the sub-carriersthat will carry the ACK/NACK messages for that user device. Thedependence between the sub-carriers used for the uplink data and thesub-carriers used for the ACK/NACK messages is preferably defined by adirect mapping function.

In one embodiment, the sub-carriers to be used to transmit each ACK/NACKmessage are determined using the following mapping function:

Position[0]=L*(i div M)+(i mod M)+Δ

where 0<=Δ<LFor j>0

Position[j]=Position[j−1]+L*N/M

where L is the number of sub-carriers in a chunk; i is the chunk numberallocated to the user device to which the ACK/NACK message is to betransmitted; M is the number of sub-carriers allocated per ACK/NACKchannel; Δ is the ACK/NACK sub-carrier position offset within a chunk;and N is the total number of chunks within the allocated bandwidth.

In an alternative embodiment, the sub-carriers to be used to transmiteach ACK/NACK message are determined using the following mappingfunction:

Position[0]=L*i+Δ

where 0<=Δ<LFor j>0 and j<M

Position[j]=((Position[j−1]+L*N/M)mod L*N) in symbol j*N _(sym) /M

where L is the number of sub-carriers in a chunk; i is the chunk numberallocated to the user device to which the ACK/NACK message is to betransmitted; M is the number of sub-carriers allocated per ACK/NACKchannel; Δ is the ACK/NACK sub-carrier position offset within a chunk; Nis the total number of chunks within the allocated bandwidth; andN_(sym) is the number of available symbols in which the sub-carriers canbe allocated.

In one embodiment, the resources used for ACK/NACK messages aresignalled to the respective user devices over their L1/L2 controlchannel which identifies the uplink resources to be used for theiruplink transmissions. This can be achieved, for example, by signallingat least one index identifying the resource(s) that will be used.

The invention also provides a communication method (that is typicallyperformed in a user device) which uses a plurality of sub-carriers, themethod comprising: receiving control data defining an allocation of saidsub-carriers; transmitting uplink data using the allocated sub-carriers;and receiving ACK/NACK messages for the transmitted uplink data; whereinsaid control data is received over a control channel using a firstsubset of said sub-carriers and said ACK/NACK messages are received onan ACK/NACK channel that is separate from said control channel using asecond different subset of said sub-carriers.

In one embodiment the receiving step receives an ACK/NACK message forthe uplink data transmitted on each chunk of sub-carriers.

In a preferred embodiment the sub-carriers on which an ACK/NACK messageis to be received are determined in dependence upon the sub-carriersallocated to the user device for transmitting said uplink data. Thisremoves the need for the station transmitting the ACK/NACK messages toinform the user device of the sub-carriers that it will use to carry theACK/NACK messages for that user device. The dependence between thesub-carriers used for the uplink data and the sub-carriers used for theACK/NACK messages is preferably defined by a direct mapping function.

In one embodiment the user device determines the sub-carriers on whicheach ACK/NACK message is to be received using the following mappingfunction:

Position[0]=L*(i div M)+(i mod M)+Δ

where 0<=Δ<LFor j>0

Position[j]=Position[j−1]+L*N/M

where L is the number of sub-carriers in a chunk; i is the chunk numberallocated to the user device to which the ACK/NACK message is to betransmitted; M is the number of sub-carriers allocated per ACK/NACKchannel; Δ is the ACK/NACK sub-carrier position offset within a chunk;and N is the total number of chunks within the, allocated bandwidth.

In another embodiment the user device determines the sub-carriers onwhich each ACK/NACK message is to be received using the followingmapping function:

Position[0]=L*i+Δ

where 0<=Δ<LFor j>0 and j<M

Position[j]=((Position[j−1]+L*N/M)mod L*N) in symbol j*N _(sym) /M

where L is the number of sub-carriers in a chunk; i is the chunk numberallocated to the user device to which the ACK/NACK message is to betransmitted; M is the number of sub-carriers allocated per ACK/NACKchannel; Δ is the ACK/NACK sub-carrier position offset within a chunk; Nis the total number of chunks within the allocated bandwidth; andN_(sym) is the number of available symbols in which the sub-carriers canbe allocated.

In one embodiment, the resources that will be used for ACK/NACK messagesare signalled to the user device over their control channel. This can beachieved, for example, by signalling an index value identifying eachresource that will be used.

The present invention also provides a communication node and a userdevice operable to perform the methods discussed above.

According to another aspect, the invention provides a communicationmethod which uses a plurality of sub-carriers, the method comprising:forming control data defining an allocation of said sub-carriers foreach of a plurality of user devices; transmitting said control data tosaid user devices; receiving uplink data from a user device; generatingan ACK/NACK message for the user device; determining one or moresub-carriers to be used to transmit the ACK/NACK message to the userdevice, in dependence upon the sub-carriers allocated to that userdevice; and transmitting said ACK/NACK message to user device on thedetermined one or more sub-carriers.

In one embodiment the determining step used a predetermined mappingbetween the allocated sub-carriers and the sub-carriers used for theACK/NACK message. In one embodiment the following mapping is used:

Position[0]=L*(i div M)+(i mod M)+Δ

where 0<=Δ<LFor j>0

Position[j]=Position[j−1]+L*N/M

where L is the number of sub-carriers in a chunk; i is the chunk numberallocated to the user device to which the ACK/NACK message is to betransmitted; M is the number of sub-carriers allocated per ACK/NACKchannel; Δ is the ACK/NACK sub-carrier position offset within a chunk;and N is the total number of chunks within the allocated bandwidth.

In another embodiment the following mapping can be used:

Position[0]=L*i+Δ

where 0<=Δ<LFor j>0 and j<M

Position[j]=((Position[j−1]+L*N/M)mod L*N) in symbol j*N _(sym) /M

where L is the number of sub-carriers in a chunk; i is the chunk numberallocated to the user device to which the ACK/NACK message is to betransmitted; M is the number of sub-carriers allocated per ACK/NACKchannel; Δ is the ACK/NACK sub-carrier position offset within a chunk; Nis the total number of chunks within the allocated bandwidth; andN_(sym) is the number of available symbols in which the sub-carriers canbe allocated.

This aspect of the invention also provides a communication method whichuses a plurality of sub-carriers, the method comprising: receivingcontrol data defining an allocation of said sub-carriers on which uplinkdata can be transmitted; transmitting said uplink data; determining oneor more sub-carriers to be used to receive an ACK/NACK message for thetransmitted uplink data, in dependence upon the sub-carriers allocatedfor transmitting said uplink data; and receiving an ACK/NACK message forthe transmitted uplink data on the determined sub-carriers. Typicallythe sub-carriers on which the ACK/NACK message is to be received will bedifferent from the sub-carriers used to transmit the uplink data and arerelated to them through a mapping function, such as the ones discussedabove.

BRIEF DESCRIPTION OF DRAWINGS

These and various other aspects of the invention will become apparent,from the following detailed description of embodiments which are givenby way of example only and which are described with reference to theaccompanying Figures in which:

FIG. 1 schematically illustrates a communication system comprising anumber of user mobile (cellular) telephones which communicate with abase station connected to the telephone network;

FIG. 2 illustrates the way in which a communication bandwidth of thebase station shown in FIG. 1 can be allocated to a number of differentmobile telephones having different supported bandwidths;

FIG. 3 illustrates the way in which sub-carriers in the downlink can bereserved for carrying the ACK/NACK information;

FIG. 4 illustrates an alternative way in which sub-carriers in thedownlink can be reserved for carrying the ACK/NACK information;

FIG. 5 illustrates a proposed control channel mapping that uses twotypes of downlink control channels of the same size;

FIG. 6 is a block diagram illustrating the main components of the basestation shown in FIG. 1;

FIG. 7 is a block diagram illustrating the main components of one of themobile telephones shown in FIG. 1;

FIG. 8 illustrates a proposed control channel mapping that uses twotypes of downlink control channels; and

FIG. 9 illustrates the way in which the ACK/NACK resource signalling canbe achieved in an alternative embodiment.

BEST MODE FOR CARRYING OUT THE INVENTION Overview

FIG. 1 schematically illustrates a mobile (cellular) telecommunicationsystem 1 in which users of mobile telephones (MT) 3-0, 3-1, and 3-2 cancommunicate with other users (not shown) via a base station 5 and atelephone network 7. In this embodiment, the base station 5 uses anorthogonal frequency division multiple access (OFDMA) technique in whichthe data to be transmitted to the mobile telephones 3 is modulated ontoa plurality of sub-carriers. Different sub-carriers are allocated toeach mobile telephone 3 depending on the supported bandwidth of themobile telephone 3 and the amount of data to be sent to the mobiletelephone 3. In this embodiment the base station 5 also allocates thesub-carriers used to carry the data to the respective mobile telephones3 in order to try to maintain a uniform distribution of the mobiletelephones 3 operating across the base station's bandwidth. To achievethese goals, the base station 5 dynamically allocates sub-carriers foreach mobile telephone 3 and signals the allocations for each sub-frameto each of the scheduled mobile telephones 3. In the proposed E-UTRA airinterface, each downlink sub-frame comprises a sequence of seven OFDMsymbols. The first two symbols typically carry the scheduling andresource allocation control data as well as other general control datawhilst the remaining five symbols contain the user data for thedownlink.

FIG. 2 illustrates an example of the way in which the base station 5 canallocate sub-carriers within its supported bandwidth to different mobiletelephones 3 having different supported bandwidths. In this embodiment,the base station 5 has a supported bandwidth of 20 MHz of which 18 MHzis used for data transmission. Typically each mobile telephone 3 isallocated one or more chunks of sub-carriers on which to transmit theiruplink data.

In order that each of the mobile telephones 3 can be informed about thescheduling decision within each sub-band, each mobile telephone 3requires a shared control channel within its camped frequency band. Thecurrent proposal for the E-UTRA air interface specifies that thiscontrol channel will include:

-   -   i) resource block allocation information (for both downlink (DL)        communications and uplink (UL) communications);    -   ii) resource block demodulation information for the downlink;    -   iii) resource block demodulation information for the uplink;    -   iv) ACK/NACK for uplink transmissions; and    -   v) timing control bits.

Therefore, given the different types of information that the controlchannel must carry, the size of the control channel will depend on theindividual mobile telephone's situation, Examples of situations thatlead to different control channel sizes are given in the followingtable:

DL UL Scheduling Scheduling ACK/ Case Information Information NACK 1 MTscheduled on UL Required Required Required and DL, and awaiting ACK/NACK2 MT scheduled on DL Required Required only, and awaiting ACK/NACK 3 MTscheduled on UL Required Required only, and awaiting ACK/NACK 4 MT notscheduled on UL Required or DL, and awaiting ACK/NACK 5 MT scheduled onUL and Required Required DL, not awaiting ACK/NACK 6 MT scheduled on DLRequired only, not awaiting ACK/NACK 7 MT scheduled on UL Required only,not awaiting ACK/NACK

The inventor has realised that having control channels of differentsizes will create problems, as either the sizes of the control channelswill have to be signalled to the mobile telephones 3 or the receivingmobile telephones 3 will have to consider all possible sizes to try torecover the control channel data. The inventor has realised that thisproblem can be avoided or at least mitigated by removing the ACK/NACKfield from the control channel itself into a dedicated (semi-static)time/frequency resource. In addition, if a mobile telephone 3 isscheduled on both UL and DL then the UL scheduling information can becontained within the allocated DL resource block. This leaves two casesfor the DL control channel size:

-   -   Type 1: DL Scheduling Information (used in cases 1, 2, 5 and 6        above)    -   Type 2: UL Scheduling Information (used in cases 3 and 7 above)

First Embodiment

The inventor proposes that one or more sub-carriers in the downlink bereserved for carrying ACK/NACK information for mobile telephones 3expecting such information in the downlink. The number of resourcesreserved for such usage and their locations in the time/frequency planecan be intimated to the mobile telephones through common signalling. Inthis embodiment, to reduce the signalling required to inform the mobiletelephones of which sub-carriers carry their ACK/NACK information, themobile telephones are programmed to work out on which sub-carriers theirACK/NACK information will be transmitted using the UL chunk allocationfor the data being acknowledged and information obtained from the commonsignalling channel. There are various techniques that can be used toperform the actual mapping between the allocated chunks for uplinktransmissions and the sub-carriers allocated for the correspondingACK/NACK messages.

First Example Mapping

In this example, the mobile telephones 3 are informed by the basestation 5 over the common signalling channel the number (M) ofsub-carriers allocated by the base station 5 to each ACK/NACK channel,with one ACK/NACK channel being used to acknowledge the data transmittedon one chunk of sub-carriers by a mobile telephone 3. Therefore, if amobile telephone 3 is allocated two chunks for uplink transmissions,then two ACK/NACK channels will be used to transmit the ACK/NACKcommands (messages) for that mobile telephone 3. In this example, thebase station 5 also informs the mobile telephones 3 what the ACK/NACKsub-carrier position offset (A) is within a chunk. Each mobile telephone3 then determines the mapping between each uplink transmitted chunknumber (i) on which it transmits data and the sub-carriers of thecorresponding ACK/NACK channel as below.

Position[0]=L*(i div M)+(i mod M)+Δ

where 0<=Δ<LFor j>0

Position[j]=Position[j−1]+L*N/M

where L is the number of sub-carriers in each chunk and N is the totalnumber of chunks in the allocated bandwidth, both of which willtypically (although not necessarily) be static for the system design andprogrammed into the mobile telephone 3 and the base station 5.Position[j] is the sub-carrier number used to transmit the jth ACK/NACKsymbol. The range of Position[j] is 0 to (L*N)−1, where L*N is the totalnumber of active sub-carriers in the system bandwidth. The range of j is0 to M−1, where M is the number of symbols in one ACK/NACK message.

FIG. 3 demonstrates the case for N=12, L=25, M=6 and Δ=0, where all theACK/NACK's are multiplexed within the second OFDM symbol of a downlinksub-frame. As shown, the multiplexing illustrated in FIG. 3 is designedto support a maximum of 12 simultaneous users within the 5 MHz band (inwhich each user is allocated one chunk) with each chunk beingacknowledged by a six sub-carrier ACK/NACK channel. The use of thesesub-carriers will obviously reduce the number of sub-carriers availablein the second OFDM symbol for the downlink control channel. However,this structure also allows support of a micro-sleep mode at the mobiletelephones 3, since a mobile telephone 3 expecting an ACK/NACK (and notscheduled to receive other downlink data) need monitor only the firsttwo OFDM symbols and then enter the micro-sleep mode.

Preferably the transmitted power of each ACK/NACK command is inverselyproportional to the number of chunks allocated the mobile telephone 3 inthe uplink, so that the total energy per ACK/NACK command is independentof the number of chunks being acknowledged.

As those skilled in the art will appreciate, M needs to be a factor of Nin order to exploit the full frequency diversity with an equally spacedACK/NACK sub-carrier distribution.

Another mechanism of the TDM mapping scheme illustrated in FIG. 3 is tospread the N&M ACK/NACK sub-carriers uniformly over the entire bandwithin the second OFDM symbol. However, if M is not a factor of L, theACK/NACK spacing will be non-uniform in this case.

Second Example Mapping

Instead of allocating the sub-carriers for the ACK/NACK channels in oneOFDM symbol, in an alternative allocation, they are allocated acrossmultiple symbols. For example, the ACK/NACK resources can be scatteredover the remaining (all but the first OFDM symbol which contains thepilot and control channels only) OFDM symbols.

In this example, the base station 5 will inform the mobile telephones 3of the number (M) of sub-carriers per ACK/NACK channel, an ACK/NACKsub-carrier position offset (Δ) within a chunk and the number (N_(sym))of available OFDM symbols, and the mobile telephones 3 will determinethe mapping between the uplink transmitted chunk number i and thecorresponding downlink ACK/NACK sub-carriers as below:

Position[0]=L*i+Δ

where 0<=Δ<LFor j>0 and j<M

Position[j]=((Position[j−1]+L*N/M)mod L*N) in symbol j*N _(sym) /M

Position[j] is the sub-carrier number used to transmit the jth ACK/NACKsymbol. The range of Position[j] is 0 to (L*N)−1, where L*N is the totalnumber of active sub-carriers in the system bandwidth. The range of j is0 to M−1, where M is the number of symbols in one ACK/NACK message.

FIG. 4 illustrates the case for N=12, L=25, M=6, Δ=0 and N_(sym)=6. Asthose skilled in the art will appreciate, with this type of mapping, thechunk bandwidth for user data is only reduced by a single sub-carrierwithin each symbol, however, the micro-sleep mode possibility isreduced. Further, in order to enable a uniform spacing of the ACK/NACKcommands in the time domain, M needs to be a factor of N_(sym).

Downlink Control Channel Size

Assuming one of the above structures for the ACK/NACK channels, thenumber of bits needed in the downlink control channel for a 5 MHzbandwidth mobile telephone 3 can be derived as follows—

Type 1 Type 2 Information bits Type indicator 1 1 DL Resource Allocation12 (bit mask) DL Resource Duration 3 DLTFCI 6 UL Scheduling Info ispresent in 1 DT, resource block UL Resource Allocation 7 (tree method)UL Resource Duration 3 UL Category 2 Information 10 Padding bits 0 2 CRC(Masked with UE ID) 10 10 Total information + CRC bits 33 33 Encodedbits (1/3 tail biting) 99 99 After rate matching 100 100 Number ofsub-carriers (QPSK) 50 50 Number of chunks 2 2

Padding bits are used in this embodiment to make the number of encodedbits the same for Type 1 and Type 2 so that the mobile telephones 3 onlyneed to perform one decoding attempt. Slightly modified structureswithout any padding bits can also be envisaged if required by thedesign.

An example of the proposed control channel mapping is shown in FIG. 5.In this figure we assume that control channels are individually coded inorder to allow efficient power control and possible beam-formingtechniques. Control channel positions are shown in the first OFDM symbolonly while the second symbol is assumed to carry pilot and additionalcontrol information. Each scheduled mobile telephone 3 is assumed tohave been allocated one control channel within 5 MHz with higherbandwidth capable mobile telephones 3 decoding multiple such channels.When possible, the frequency position of the control channel should bechosen to span the resources (sub-carriers) on which the user data isscheduled in order to exploit the superior channel characteristics atthese frequency positions. FIG. 5 shows a case when a maximum of twelvepossible users are scheduled within 10 MHz. In case the number of usersis less, some of the control channel resources can be freed and occupiedby user data. The absence of a control channel in a specific positioncan be indicated using a single bit field in the preceding controlchannel.

As shown in FIG. 5. Type 1 and Type 2 control channels are each assumedto span 2 chunks. The total number of control channels possible dependson the mapping adopted for the ACK/NACK channels which has not beenshown in the figure.

The structure of the ACK/NACK resource allocation can be furthersimplified by allocating only mobile telephones 3 without a downlinkresource allocation within the same sub-frame. A mobile telephone 3 witha downlink scheduling message in the same sub-frame can be intimatedabout ACK/NACK's within the downlink resource block (user data). In sucha case, a single bit ACK/NACK will suffice since the control informationwithin the downlink resource block will have its own error codingprotection. However, an error in the control channel detection in thiscase will also lead to the inability of the mobile telephone 3 frombeing able to retrieve ACK/NACK information which may, in turn, puttighter performance requirements on the downlink control channel.

Base Station

FIG. 6 is a block diagram illustrating the main components of the basestation 5 used in this embodiment. As shown, the base station 5 includesa transceiver circuit 21 which is operable to transmit signals to and toreceive signals from the mobile telephones 3 via one or more antennae 23(using the above described sub-carriers) and which is operable totransmit signals to and to receive signals from the telephone network 7via a network interface 25. The operation of the transceiver circuit 21is controlled by a controller 27 in accordance with software stored inmemory 29. The software includes, among other things, an operatingsystem 31 and a resource allocation module 33. The resource allocationmodule 33 is operable for allocating the sub-carriers used by thetransceiver circuit 21 in its communications with the mobile telephones3. The software also includes an ACK/NACK module 35, which is operablefor informing the mobile telephones 3 of the information needed to mapbetween the allocated chunk numbers for their uplink transmission to theACK/NACK channels used for the acknowledgments of that data. TheACK/NACK module 35 is also operable to transmit the ACK/NACK commandsfor the received data on the corresponding ACK/NACK channels forreception by the mobile telephones 3.

Mobile Telephone

FIG. 7 schematically illustrates the main components of each of themobile telephones 3 shown in FIG. 1. As shown, the mobile telephones 3include a transceiver circuit 71 that is operable to transmit signals toand to receive signals from the base station 5 via one or more antennae73. As shown, the mobile telephone 3 also includes a controller 75 whichcontrols the operation of the mobile telephone 3 and which is connectedto the transceiver circuit 71 and to a loudspeaker 77, a microphone 79,a display 81, and a keypad 83. The controller 75 operates in accordancewith software instructions stored within memory 85. As shown, thesesoftware instructions include, among other things, an operating system87 and a resource allocation module 89. In this embodiment, they alsoinclude an ACK/NACK module 91 that is operable to perform theappropriate mapping to identify the sub-carriers that carry the ACK/NACKcommands for the data that the mobile telephone 3 has transmitted. Themobile telephones 3 may be programmed to be able to perform only one ofthe mappings discussed above (with reference to FIGS. 3 and 4) or if thebase station 5 varies the mapping that it uses, then the mobiletelephones 3 will have to be informed of the mapping to be used for agiven sub-frame.

In the above embodiment, the resources used for the ACK/NACK messagesare related to the resources allocated to the mobile telephones 3 foruplink transmissions through an appropriate one-to-one mapping. However,the disadvantage with this approach is that if one mobile telephone 3 isallocated multiple uplink resources, then the same number of resourcesmust be used in the downlink for the ACK/NACK messages, and this is notan efficient use of the system's resources.

Second Embodiment

The above embodiment was first described in the applicant's earlierBritish patent application GB0612228.7. Since the filing of thisapplication, a number of changes have been made to the proposed E-UTRAair interface. Some of the terminology has changed, so that now asub-frame is the same as a Transmission Time Interval (TTI) andcomprises two 0.5 ms slots, each of which comprises the above describedseven OFDM symbols. Also a resource block (RB) or a chunk consists of 12consecutive sub-carriers in the frequency domain. Additionally, in thecurrent proposal each base station 5 will support only one bandwidth ata time, but it can be upgraded to other bandwidths up to the 20 MHzmaximum bandwidth. The mobile telephones 3 that communicate with thebase station 5 will all have to support the same bandwidth as the basestation 5.

The proposal for the L1/L2 control channel structure (over which theresource allocations are signalled) has also changed. In particular, thecurrent proposal, illustrated in FIG. 8, is to reserve a certain amountof time-frequency resources for downlink signalling in the first n OFDMsymbols, where n≦3 and it is assumed that the second OFDM symbol willcarry the ACK/NACK resources. Each scheduled mobile telephone 3 isassumed to have been allocated one or more control channels within theoperating bandwidth of the base station 5 (in this example 10 MHz). Theavailable resources are divided into a number of “control channelelements” (CCEs) of uniform size. A control channel for a mobiletelephone 3 can be formed from one of these CCEs or from a number ofaggregated CCEs. The more CCEs that are used for one control channel thegreater the coding gain that can be achieved, so more CCEs will tend tobe used for users with worse channel conditions (eg for users at theedge of the cell). When possible, the frequency position of the controlchannels should be chosen to span the resources on which the user datais scheduled in order to exploit the superior channel characteristics atthese frequency positions or spread across the whole bandwidth to getlarge frequency diversity. The mapping of the CCEs to the controlchannels is dynamic and controlled by the base station 5 on a sub-frameby sub-frame basis. A mobile telephone 3 is told a set of CCEs tomonitor in case it is being sent a scheduling message. The CCEaggregation is unknown to the mobile telephone 3, so it must trydecoding each CCE on its own, then pairs of CCEs together, etc. If thedecoding works then it knows that it has found the correct combinationand can read the message. Separate control channels may also be providedfor each mobile telephone 3 for downlink and uplink resource scheduling.

With this arrangement, the ACK/NACK resources that are used could bedefined in a similar way to the first embodiment, but with reference tothe resources used to define the downlink (L1/L2) control channel,rather than the allocated uplink resources. However, this approachrequires each mobile telephone 3 to know the index of the resources usedfor its L1/L2 downlink control channel relative to those of the othercontrol channels. However, with the current proposal, each mobiletelephone 3 only knows that it correctly decoded its L1/L2 controlchannel. It does not know the index of the resources used for itscontrol channel relative to those of other mobile telephones 3.

Therefore, in this second embodiment, the index of the ACK/NACKresources that will be used by the base station 5 are signalled inadvance to the mobile telephone 3 within its L1/L2 control channel usedfor uplink resource allocation. This process is illustrated in FIG. 9.As shown, the base station 5 signals the mobile telephone 3 over theL1/L2 control channel used for uplink assignment with the index of theACK/NACK resources that will be used by the base station 5 to signal theACK/NACK messages to the mobile telephone 3 after it has transmitted itsunlink data.

With this arrangement, there is also no need to create separateresources for dynamically scheduled mobile telephones 3 and persistentlyscheduled mobile telephones 3. In both cases, a pool of resources is putaside for ACK/NACK transmissions for all mobile telephones 3. Then eachmobile telephone 3 expecting an ACK/NACK response is signalled an indexcorresponding to its intended ACK/NACK resources. As those skilled inthe art will appreciate, the number of bits required to signal the indexwill depend upon the number of resources reserved as ACK/NACK resources.Additionally, if more than one ACK/NACK resource is required, then morethan one index may be inserted into the L1/L2 control channel.

Modifications and Alternatives

A number of detailed embodiments have been described above. As thoseskilled in the art will appreciate, a number of modifications andalternatives can be made to the above embodiments whilst stillbenefiting from the inventions embodied therein. By way of illustrationonly a number of these alternatives and modifications will now bedescribed.

In the above embodiments, a mobile telephone based telecommunicationsystem was described in which the above described ACK/NACK resourcesignalling techniques were employed. As those skilled in the art willappreciate, the signalling of such ACK/NACK resources can be employed inany communication system that uses a plurality of sub-carriers. Inparticular, the signalling techniques described above can be used inwire or wireless based communications either using electromagneticsignals or acoustic signals to carry the data. In the general case, thebase station would be replaced by a communication node whichcommunicates with a number of different user devices. User devices mayinclude, for example, personal digital assistants, laptop computers, webbrowsers, etc.

In the above embodiments, the base station was assumed to have anoperating bandwidth of 20 MHz in the first embodiment and 10 MHz in thesecond embodiment and the chunks of carrier frequencies were defined tocomprise 25 sub-carriers each. As those skilled in the art willappreciate, the invention is not limited to these particular chunk orbandwidth sizes.

In the above embodiments, a number of software modules were described.As those skilled will appreciate, the software modules may be providedin compiled or un-compiled form and may be supplied to the base stationor to the mobile telephone as a signal over a computer network, or on arecording medium. Further, the functionality performed by part or all ofthis software may be performed using one or more dedicated hardwarecircuits. However, the use of software modules is preferred as itfacilitates the updating of the base station 5 and the mobile telephones3 in order to update their functionalities.

The following is a detailed description of the way in which the presentinventions may be implemented in the currently proposed 3GPP LTEstandard. Whilst various features are described as being essential ornecessary, this may only be the case for the proposed 3GPP LTE standard,for example due to other requirements imposed by the standard. Thesestatements should not, therefore, be construed as limiting the presentinvention in any way. The following description will use thenomenclature used in the Long Term Evolution (LTE) of UTRAN. Forexample, a base station is referred to as eNodeB and a user device isreferred to as a UE.

1. Introduction

In the previous RAN1#48 meeting, the following working assumption wasagreed for ACK/NACK control signalling and relevant pre-configuredresources [1]:

-   -   The resources used for ACK/NACK are configured on a semi-static        basis        -   Defined independently of the control channel format    -   Implicit relation between the uplink resources used for        dynamically scheduled data transmission, or the DL control        channel used for assignment, and the downlink ACK/NACK resource        used for feedback.

However, the last bullet point does not clearly indicate how theACK/NACK is signaled to a specific UE.

In this document, we analyse the existing signalling options forACK/NACK control signalling and propose an efficient signallingmechanism for ACK/NACK for each UE.

2. Downlink ACK/NACK Control Signalling

NodeB sends the ACK/NACK information in response to uplink transmissionreceived from the UE. Subsequently, UE expects its ACK/NACK informationin one of the pre-configured downlink resources. The assumption is thatthere are a number of subcarriers in the downlink that are reserved forcarrying ACK/NACK information for all UEs who are expecting suchinformation in the downlink. The number of resources reserved for suchusage and their locations in the time/frequency plane can be informed toall UEs in the cell through common signalling in semi-static basis.However, if UE expects ACK/NACK information it needs to know where tolook for its ACK/NACK information in these reserved resources.

In RAN1, UE ID-less ACK/NACK signaling has been proposed in order toreduce the signaling overhead [2-6]. It is proposed an implicitsignalling for UE where to find its ACK/NACK information in thesereserved resources.

3. Implicit ACK/NACK Signalling

Within the implicit signalling, there may be at least two options:

-   -   Option 1: Implicit relation between the uplink resources used        for dynamically scheduled data transmission and the downlink        ACK/NACK resource used for feedback.    -   Option 2: Implicit relation between the DL control channel used        for assignment and the downlink ACK/NACK resource used for        feedback:        -   One-to-one relationship between the index of the downlink            L1/L2 control channel for uplink radio resource assignment            and the index of ACK/NACK radio resources.

Option 1 assumes that the number of ACK/NACK resources is equal to thenumber of uplink resource blocks (RBs) so that there is a relationshipbetween them. UE knows where to expect the ACK/NACK information and itcan work out from knowledge of the UL resources used for the ULtransmission on which sub-carriers the ACK/NACK information will betransmitted. However, the disadvantage with Option 1 is that if one UEis allocated multiple RBs in the uplink, then there are same number ofACK/NACK resources corresponding to these RBs. It is not efficient thatNodeB to signal one ACK/NACK information to all these resources. Hence,Option 1 wastes some downlink resources.

Option 2 assumes one-to-one relationship between the index of thedownlink L1/L2 control channel for uplink radio resource assignment andthe index of ACK/NACK radio resources. The disadvantage with Option 2 isthat UE does not know its index relative to the other downlink L1/L2control channel for uplink radio resource assignment. UE only knows thatit correctly decoded its L1/L2 control channel for uplink radio resourceassignment.

In the last way forward agreement [1], it was agreed that the controlchannels are formed by aggregation of control channel elements (CCE).The assumption is that each UE knows its MCS format so that it can trysome decoding attempts blindly to find its downlink L1/L2 controlchannels. If UE decodes its downlink L1/L2 control channel for uplinkradio resource assignment, then it knows the indices of the assignedcontrol channel elements (CCE) relative to all other CCEs in thebandwidth. So, it is possible that UE uses the Index of the CCE.However, there is large number of CCEs in the bandwidth and UE may beassigned one or more CCEs. Then, this Option 2 has similar disadvantageas in Option 1, hence, it is not efficient.

4. Index Signalling in the DL L1/L2 Control Channel

The disadvantage of the Option 2 can be avoided by signalling the indexof the ACK/NACK resources to the UE in advance so that it knows where toexpect ACK/NACK information relative to the other UEs. In this case, theindex is inserted in the DL L1/L2 control channel for uplink radioresource assignment as shown in FIG. 8. The number of bits for indexingdepends on the number of resources reserved for ACK/NACK resources ineach bandwidth.

In our proposal, there is no need to create separate resources fordynamically scheduled UEs and persistently scheduled UEs. In both cases,a pool of resources is put aside for Ack/Nack transmissions for all UEs.Then each UE expecting ACK/NACK response is signalled an indexcorresponding to its intended ACK/NACK resources.

6. Conclusions

In this document, we have analysed the existing signalling options forACK/NACK control signalling and show the drawbacks of the existingoptions. In addition, we have proposed a signalling mechanism thatavoids the drawbacks of the existing options by inserting an index inthe DL L1/L2 control signalling for uplink radio resource assignment.Hence, we propose:

-   -   The index inserted in the downlink L1/L2 control channel for        uplink radio resource assignment must be used for ACK/NACK radio        resources.

6. References

-   [1] R1-071223, “Way Forward on Downlink Control Signaling” Ericsson,    Nokia, NTT DoCoMo, et al.-   [2] R1-070867, “ACK/NACK Signal Structure In E-UTRA”, NTT DoCoMo, et    al.-   [3] R1-070932, “Assignment of Downlink ACK/NACK channel”, Panasonic.-   [4] R1-070734, “ACK/NACK Channel Transmission in E-UTRA Downlink”,    TI-   [5] R1-070791, “Downlink Acknowledgement and Group Transmit    Indicator Channels”, Motorola

What is claimed:
 1. A user device which is operable to communicate witha communication node, the user device comprising: a receiving circuitconfigured to receive control data defining an allocation of at leastone resource block for use in transmitting uplink data; a transmittingcircuit configured to transmit the uplink data using the at least oneresource block for use in transmitting uplink data; and a processorconfigured to determine at least one sub-carrier on which at least oneACK/NACK message is to be received based on the at least one resourceblock for use in transmitting uplink data; wherein the receiving circuitis configured to receive the at least one ACK/NACK message over anACK/NACK channel using the at least one sub-carrier.
 2. A methodperformed by a user device which is operable to communicate with acommunication node, the method comprising: receiving control datadefining an allocation of at least one resource block for use intransmitting uplink data; transmitting the uplink data using the atleast one resource block for use in transmitting uplink data;determining at least one sub-carrier on which at least one ACK/NACKmessage is to be received based on the at least one resource block foruse in transmitting uplink data; and receiving the at least one ACK/NACKmessage over an ACK/NACK channel using the at least one sub-carrier. 3.A communication node which is operable to communicate with a userdevice, the communication node comprising: a transmission circuitconfigured to transmit control data defining an allocation of at leastone resource block for use in transmitting uplink data from the userdevice; a receiving circuit configured to receive the uplink data; and aprocessor configured to determine at least one sub-carrier based on theat least one resource block for use in transmitting uplink data from theuser device, wherein the transmission circuit is configured to transmitat least one ACK/NACK message over an ACK/NACK channel using the atleast one sub-carrier.
 4. A method performed by a communication nodewhich is operable to communicate with a user device, the methodcomprising: transmitting control data defining an allocation of at leastone resource block for use in transmitting uplink data from the userdevice; receiving the uplink data; determining at least one sub-carrierbased on the at least one resource block for use in transmitting uplinkdata from the user device; and transmitting at least one ACK/NACKmessage over an ACK/NACK channel using the at least one sub-carrier.